Radio repeating system



Nov. 2l, 1950 H. T. wlNcHx-:L

RADIO REPEATING SYSTEM 2 Sheets-Sheet 1 Filed Aug. 17, 1945 ATTORNEY Ngv. 21, 1950 H. T. wlNcHEL RADIO REPEATING SYSTEM *Patented Nov. 21, 1950 RADIO REPEATING SYSTEM Henry '.l. Winchel, Roscoe, Calif., assignor to Bendix Aviation Corporation, South Bend, Ind., a corporation of Delaware Application August 17, 1945, Serial No. 611,010

(Cl. Z50- 15) 8 Claims.

This invention relates to radio communication systems of such extent, relative to the range and character of the wave energy employed, that repeater stations are necessary, and it is particularly useful in systems for radio communication between vehicles operating over areas of considerable extent.

An object of the invention is to provide a radio repeating system that is simple and practicable, for extending the range of low power high frequency signals.

Another object is to provide a radio repeating system that will enable two-way communication over an area of indefinite extent with the use of carrier waves of only two frequencies.

Another object is to provide a system that adapts high frequency radio waves having straight 'line propagation characteristics for practical use in communication between vehicles and the like.

Other more specific objects and features will become apparent from the description to follow, of a system in accordance with the invention.

As adapted for two-way communication between vehicles traversing an area, or between vehicles and fixed stations, the present invention employs a plurality of repeating stations distributed across the area at intervals somewhat less ythan the positive range of the signals transmitted, which repeaters function to pick up signals from any part of the area and repeat them over the entire area. The communicating stations (hereinafter referred to as sub-stations), which may be located either on vehicles traveling over the area or at fixed locations within the area, are capable of receiving signals on either or both of two frequencies f1 and f2, but need transmit on only one frequency, f2. The repeating stations all have substantially the same equipment and function to repeat the signals on the other frequency than that on which the signals are received. Thus, signals transmitted from a substation on frequency f2 are received by the nearest repeating station and retransmitted on frequency f1, which may be received directly by another sub-station, and is also received by one or both of the next adjacent repeating stations, which in turn retransmit the signals on frequency f2. The repeating continues from one repeating station to the next with a shift in frequency at each repeating station, to the end of the system.

A system, as so far described, would be subject to undesired oscillation because of singing circuits set up between the different repeating stations, if provision were not made to prevent it.

In accordance with the present invention singing circuits are prevented by employing directional reception and transmission in part at the repeating stations. Thus, each repeating station includes the equivalent of three separate repeating units so interlocked that when any one of them is functioning, the other two are locked out. One of the units receives directionally from one direction on frequency f1 and transmits directionally in another direction on frequency f2. The second unit receives directionally on frequency f1 from the said other direction and transmits directionally on frequency f2, in the said one direction. The third unit receives non-directionally on frequency f2 and transmits non-directionally on frequency f1.

When a sub-station transmits, on frequency f2, its signals are received non-directionally at the nearest repeating station and retransmitted nondirectionally on frequency f1. However, the signals retransmitted on frequency f1 are received by the next adjacent repeating stations directionally and are retransmitted on frequency f2, in direction away from the rst mentioned repeating station, so that they cannot be received at the first mentioned repeating station with sufficient intensity to set up a singing circuit, or loop, between two repeating stations.

The operation as described is repeated throughout the system, one set of alternate repeating stations retransmitting the signal non-directionally on frequency f1 and the other set of alternate repeating stations retransmitting the signals directionally on frequency f2.

Referring now to the drawing:

Fig. 1 is a plan View, or map, showing typical vlocations of signaling stations and of repeating stations in a system in accordance with the invention;

Fig. 2 is a schematic diagram showing the essential equipment elements at each of the stations in the system of Fig. 1;

Fig. 3 is a schematic diagram showing in greater detail. than does Fig. 2, the equipment at each repeater station;

Fig. 4 is a detailed plan view of a small portion of the system of Fig. l showing the use of a booster repeater station;

Fig. 5 is a schematic diagram showing the essential equipment elements at the booster station of Fig. 4;

Fig. 5 is a diagram illustrating an allocation of frequencies that may be employed at the repeating stations in the system;

.Fig 7 iS a Schematic diagram of one type of U receiving circuit that can ce employed at the substations; and

Fig. 8 is a schematic diagram showing another type of receiving circuit that can be employed.

General description of system Referring rst to Fig. l, there is shown a system in accordance with the invention for pro- Viding communication between a plurality oi suhstations located at intervals along a highway ES, some of the sub-stations consisting of mobile units mounted on Vehicles adapted to operate over the road 2t, and other sub-stations being located at fixed points along the road.

Thus, there are shown two terminal sub-stations X and Y located at opposite ends of the road'Zt, which may be of any length. There are also shown two intermediate stationary suhstations Si and S2, which are positioned adi the road lout at spaced intervals therealong, intermediate the terminal stations X and Y. There are also shown three mocile suo-s tations i, T15?, and M3, which loe on busses, trucks, police cars, or other vehicles operating on the road It is contemplated thatA the system of l will employ radio waves of relatively high frequencies capable of being propagated only along a line of sight (disregarding reflections) and of such power as to have a range much less than the total length of the road 23, so that the signals will have to he repeated at intervals along the route in order to effect proper coverage. To this end, repeater stations are provided at hired points along the route, three such repeater stations identified as Rl, R2, and being shown in Fig. 1.

As previously pointed out, the present invention provides thorough coverage of the entire route while employing only two frequencies, and without the necessity of manual switching for the selection of' frequencies. This result is ohtainedloyv making thev receivers at all the suhstations responsive to both frequencies at all times, and byl changing the frequency at each peater station, and using directional trai? irission andreception at some of the repeater stations.

It is preferable, although not essential., to utilize either uni-directional or 1.ai-directional transmission and reception at the fixed sub-stations, while using non-directional transmitting andV receiving equipment at the mobile sub-stations'.

In Fig. l, the held patternsof' transmission and reception at each xed station are roughly indicated'by sectors. Thus, the iixed sub-station X receives and transmits with maximum efciency v/ithinthe sector 2 l. The repeater station Rl receives and transmits with maximum efliciency in the two ectors 22 and 23, respectively; the repeating station R2 receives and transmits with maximum efciency in the two sectors ld and 25, respectively; and the receiving station transmits and receives with maximum efficiency in the two sectors 25 and 2l, respectively. terminal sub-station Y receives and transmits with maximum eliiciency within the sector The equipment at the diiferent stations is shown schematically in Fig. 2. Thus, the equipment at the terminal sub-station X comprises a transmitter ii@ having a directive antenna indicated as an arrow pointed in the direction to effect coverage within the sector 2i of Fig. l, and a receiver @el having a directive. antenna Si indicated as an arrow so pointed as to receive with CA i maximum efliciency signals transmitted at points within the sector 2i. The transmitter 5i: transmits on one frequency identied as f2, and the receiver is adapted to receive signals on both the frequency f2 and a different frequency fl. Two forms of receiver capable of simultaneously receiving signals on two properly chosen frequencies will be described later with reference to Figs. '7 and 8.

The mobile sub-station Ml is shown provided with a transmitter 32 adapted to transmit signals on the frequency f2 over a non-directional transmitting antenna 33, and a receiver 355 having a non-directional receiving antenna 35 and adapted to receive signals on both frequencies fi and f2.

The apparatus aty all the repeating stations, Rl, R2, and R3, is identical except with respect to the orientation of the antennas. Thus, the apparatus at repeating station Rl comprises a repeater S8 having a uni-directional receiving antenna 3l adapted to receive signals entering from sector at frequency fi, and a uni-directional transmitting antenna over which the same signals are repeated on frequency f2 in to sector 22. It includes a second repeater 39 having a uni-directional receiving antenna 4l] adapted to receive signals on frequency f!` from sector 22, and having a uni-directional transmitting antenna ii from which the same signals ar retransmitted on frequency f2 into sector 23. Repeating station Rl includes a third repeater l2 having a loi-directional antenna t3 adapted to receive signals on frequency f2 from either sector 22 or sector 23 and retransmit the received signals from a loi-directional transmitting antenna lf2- on frequency fi into both sectors 22 and 23.

As indicated diagrammatically in Fig. 2, the three repeaters 39 and c2 at each repeating station are interlocked. The interlock will be more fully explained later with respect to Fig. 3, and it will suiilce to state at this point that the interlock functions to disable any two of the repeaters while the third repeater is in operation. In other words, during reception of a signal at frequency fl by the repeater 36 over its receiving antenna 3l, the repeaters 3S and 42 are disabled; while the repeater 39 is repeating signals rceved at frequency fl over its receiving antenna Mi, the repeaters t5 and l2 are disabled; and while the repeater 32 is repeating signals received on frequency f2 over its receiving antenna d3, the repeaters 35 and 3@ are disabled. This interlock is an important feature of the invention and is essential to the proper operation of the system.

The apparatus at the stationary sub-station l includes a receiver 415 having a loi-directional receiving antenna adapted to receive signals on both frequencies fi and f2 from both directions along the route within sectors Hl? and III. It also includes a transmitter fil having a bi-directional transmitting antenna 48 from which signals may be transmitted on frequency f2 in either direction along the route.

The apparatus at the repeating station R2 is identical with that at the repeating station RI except for the orientation of the antennas, and corresponding parts hear the same reference numerals. It will he observed that at station R2 the repeater E@ receives on frequency fl from sector 25 and retransmits on frequency f2 to sector 2li; repeater 3S receives signals on frequency ji from sector Ell and retransmits on frequency Af2 to sector 25; and the repeater 42 receives signals on frequency f2 from both sectors 24 and 25 and retransmits the signals at the frequency fl into the same sectors 24 and 25.

The apparatus at the mobile sub-station M2 is identical with that at the mobile sub-station Ml, and corresponding parts bear the same reference numerals.

The apparatus at repeating station R3 is identical with that at repeating stations Rl and R2 except for the orientation of the antennas, and corresponding parts bear the same reference numerals. It will be observed that the repeater 3S receives signals on frequency fl from sector 21 and retransmits them on frequency f2 to sector 26; repeater 39 receives signals on frequency fl from sector 26 and retransmits them on frequency f2 to sector 2l; and the repeater 42 receives signals on frequency f2 from either sector 26 or sector 2l and retransmits the signals on frequency fl into both those sectors, 26 and 2T.

The apparatus at the stationary sub-station S2 is identical with that at the sub-station Sl except for the orientation of the directive antennas, and corresponding parts bear the same reference numerals. It receives and transmits within the sectors |2 and I3.

Likewise, the apparatus at the mobile substation M3 is identical with the mobile stations M| and M2, and corresponding parts bear the same reference numerals.

rEhe apparatus at the terminal sub-station Y is identical with that at the terminal sub-station X except for the orientation of the directional antennas, and corresponding parts bear the same reference numerals. It will be observed that the receiver 3|) is adapted to receive signals on both frequencies f| and f2 in sector 28, and the transmitter 5B is adapted to transmit signals on frequency ,fl to sector 28.

The antennas 29 and 3| at the terminal stations X and Y are shown as uni-directional, and the antennas 43 and 44 at the repeating stations RI, R2, and R3, and the antennas 46 and 48 at the stationary sub-stations Sl and S2 have been shown as bi-directional. However, it is not essential to the proper operation of the system that these antennas 29, 3|, 43, 44, 46, and 48 be directional, and they can be non-directional. The only advantage of making them directional, as shown, is to improve the efficiency of transmission Iand reception in the useful directions. Whether or not they transmit and receive efciently in other directions is immaterial.

However, it is essential to the proper operation of the system that the antennas 3l, 38, 40 and 4| at each repeating station be uni-directional, in order to prevent setting up singing circuits in the system. The reason for this will become apparent from the description of operation of the system to follow.

Operation when transmitting from sub-station X The system described functions to blanket the entire route with signals originating at any one of the sub-stations. In some regions, the signal will be of useful intensity only at the frequency fl, and in other regions only at the frequency f2. However, by virtue of the fact that the receivers at all the sub-stations are adapted to respond to signals of both frequencies, every station will hear a signal transmitted from any other station.

Assume that a signal is transmitted on frequency f2 from the terminal sub-station X. The

Asignal is transmitted on frequency f2 throughout sector 2| and will be received by the mobile substation MI, and by the repeating station RI. At the repeating station R|, the signal is received on the bi-directional antenna 43 and is retransmitted over the loi-directional antenna 44 at fre-1 quency fl into sectors 22 and 23. In sector 23; it will be received by the stationary station Sl and the repeating station R2. The stationary station SI may also be receiving the signal simultaneously on frequency'fZ directly from the originating station X, but this is immaterial.

The repeating station R2 receives the signals from repeating station Rl on frequency fl over its receiving antenna 43 and retransmits the signals on frequency f2 forwardly into sector 24.

The signals transmitted on frequency f2 from repeating station R2 must not be received by the antenna 43 at repeating station RI, else a sing-l ing circuit would be set up, and it is for this reason that antenna 4| is made uni-directional and directed toward repeating station R3. Of course, it is impracticable to provide antennas that have perfect directional characteristics, and weak signals from the antenna 4| at repeating station R2 will be received on antennal 43 at repeating station Rl, but the magnitude of the energy received at repeating station RI from antenna 4| at repeating station R2 can be readily kept small enough to prevent a singing circuit being set up.

As previously indicated, the repeating station R2 retransmits the signals which originated at station X into the sector 25 at a frequency of f2, which signals will be received by the mobile substation M2 and the repeating station R3.

At repeating station R3, the signals will be received on the bi-directional antenna 43 and retransmitted by the bi-directional antenna 44 at frequency f| both back into sector 26 and forward into the sector 2T, where they will be received by thestationary station S2, the mobile sub-station M3, and the terminal sub-station Y. Because of the uni-directional characteristics of the antenna `4|) at repeating station R2, the sig-l nals transmitted back into sector 26 will not be received at repeating station R2 with sufficient intensity to set up a singing circuit.

It will be observed from the foregoing description of operation that in response a transmission of signals from the terminal station X on frequency f|, the signals are repeated throughout the systemen either frequency f| or f2.

It is important to note that, during the operations described, the repeaters 36 and 39 at repeating station R| are disabled by the interlock system while the repeater 42 is in operation. This prevents signals that are being transmitted from repeating station R2 on frequency fi from being received and retransmitted by the repeater 36 at repeating station Rl and prevents repeater 39 from repeating the signals being transmitted from the local antenna 44. Likewise at repeating station R2, While the repeater '39 is retransmitting, the repeater 36 is locked out to prevent it from retransmitting signals on frequency `f2 in response to the signals at frequency f| that are simultaneously being transmitted from the local repeater 39, and the repeater 42 is locked out to prevent it from responding to the signals on frequency f2 from repeating station R3.

t the repeating station R3 the repeaters 316 and 3S are locked out while the repeater 42` is in operation, to prevent the repeaters 36 and 39 from responding to the signals of frequency fl being transmitted by repeater 42.

Transmission from mobile station M1 As shown in Fig. 1 the mobile sub-station MI is located between the terminal sub-station X and the repeating station RI, and if the operator at station Ml wishes to communicate with the rest of the system, he transmits from the nondirectional antenna 33 on frequency f2. His signal is picked up directly by the antenna 3l at the terminal station X, and may also be picked up directly by the stationary sub-station SI, but it is immaterial whether or not station Sl receives the signals directly. The signals are also, of course, picked up at repeating station RI by the antenna 43 feeding into the repeater 42, which retransmits the signals in both directions along the route on frequency fl from the antenna 44. The signals are repeated beyond repeating station RI exactly the same as described in connection with signals originating at station X. The entire area covered by the system will therefore be blanketed with signals emanating from the mobile sub-station MI, either on frequency ,fl or frequency f2, and will be picked up on one or the other of the two frequencies at all of the stations.

Transmission from mobile station M2 Assume now that a signal originates at the mobile sub-station M2, which at the time is located approximately mid-Way between the repeating stations R2 and R3. The transmitter 32 transmits non-directionally from the antenna 33 (Fig. 2) lon frequency f2 and the signals are received on the antennas 43 at both repeating stations R2 and R3, and retransmitted non-directionally by both those stations on frequency fl. The signals transmitted on frequency fl from either repeating station R2 or R3 do not affect the other, because at both stations the units 36 ,and 39 which are responsive to frequency fl are locked out. However, the signals transmitted from repeating station R2 on frequency fl are received at repeating station Rl on antenna 31 and retransmitted by unit 36 on frequency f2 over vantenna 38. Likewise, the signals retransmitted from antenna 44 on frequency fl at repeating station R3 are received by the sub-stations S2, M3 and Y.

It is believed that with the foregoing explanai tion, the operation of the system can be readily followed regardless of the particular station in the system at which signals originate.

Apparatus at repeating stations One type of apparatus that may be provided at each of the repeating stations will now be described with reference to the schematic diagram of Fig. 3, in which the same reference numerals employed in Fig. 2 designate corresponding parts. It will be observed that the apparatus within all .three units 36, 39 and 42 is substantially identical, the only difference being in the frequencies to which the amplifying and oscillating elements ar tuned. 1

Thus, in unit 42, signals of frequency f2 received on the antenna 43 are ampliiied in an amplifier V`55, detected in a detector 56, and applied to a resistor 58, and the voltage across resistor 58 is applied through a resistor 59 to the s grid 6,0 of a tricdel the grid being connected to the cathode 62 and ground through a condenser 63. The anode 64 of the triode is connected through `the winding of a relay 51 in unit 42, the winding of a relay 65 in unit 39 and the winding 8 of a relay 65 in unit 36 to a source of anode potential 66.

The detected audio potentials developed across resistor 58 are also applied through a condensor 61 to a modulator 69. An oscillator 10 supplies carrier current of frequency f I to the modulator 69, when the oscillator is energized, and the modulated current of frequency fl is delivered to the transmitting antenna 44.

Normally, When no signals are received, the grids 69 of the tubes 6l in all three of the units 36, 39 and 42 are substantially at the same potential as their cathodes, under which conditions the anode current of each tube is suflicient to energize the relay 51 in the same unit and the relay 65 or the relay 13 in each of the other units. Thus, it will be observed that current from the anode 64 of the tube 6| in unit 42 flows through the local relay 51 through the relay 65 in unit 39, and through the relay 65 in unit 36. The anode current from the tube 6l in unit 39 flows through the winding of local relay 51, through the Winding of relay i3 in unit 36, and through the Winding of relay 65 in unit 42. The anode current of the tube 6l in unit 36 flows through the winding of the local relay 51, through the lWinding of relay 73 in unit 39, and through the winding of relay 'i3 in unil*l 42. Hence, when the repeating station is in a standby condition all of the relays 51, 65, and i3 are energized. When the relays 51 are energized their contacts 51| are opened to interrupt the supply of anode current to the oscillators 1E! so that no signals are being transmitted. However, as long as both the relays 65 and 13 in each unit are energized they close their contacts 1 and 'i2 respectively to supply anode potential to the receiving amplier 55, so that all three units are ready to respond to received signals.

When a signal is received by any one of the units, the detected signal develops a negative potential, with respect to ground, across the resistor 58, which is applied through the resistor 59 to the grids 66 of the tubes 6I in that unit and reduces the anode current of that tube to a small v value insuflicient to hold the relays connected between the anode of that tube and the anode supply source 65. The release of the local relay 51 closes the contacts A51| to supply anode potential to the local oscillator 1I] to retransmit the received signals. On the other hand, the release of the relay 65 or 13 in each of the other units opens the contacts 1I or 12 to interrupt the supply of anode potential to the receiving amplifiers 55 in those units, thereby making them non-responsive to received signals.

Thus, assume that While the apparatus in all three units, 38, 39, and 42 is in standby condition, with all of the relays 51, 65, and 13 energized, a signal is received on the antenna 43 of unit 42. This signal is amplified in the amplifier 55, detected in the detector 56 and the detected signal develops a potential across the resistor 58 that is applied through the resistor 59 to the grids 6D of tubes 6l and through the condensor 61 to the modulator 69. The resultant negative potential on the grids 66 reduces the anode current of tubes 6I, causing the local relay 51 in unit 42 and the relays 55 in units 39 and 36 to release. The release of relay 51 in unit 42 closes the contacts 51| to energize the oscillator 19 and cause unit 42 to retransmit over antenna 44 the signals being received on the receiving antenna 43. However, the release of relays 65 in units 39 and 36 interrupts the supply of anode potential to the Y vreceiving amplifiers 55 in units 39 and 36 and ataque .9. renders them incapable of responding to received signals while the repeating unit 42 is in operation.

summarizing, it will be observed that the first of the three units 36, 39 and 42 to receive a signal will respond to energize its oscillator to repeat the received signal, while disabling the receiving amplifiers 55 of the other two units and locking them out until the message being received by the rst unit is terminated.

Miscellaneous features It is to be understood that, because of limitations on size and weight of the equipment mounted on vehicles, the transmitters at the mobile stations Ml, M2-and M3 are desirably of relatively low power. This is permissible in the present system, because the mobile transmitters need only have suilcient power to positively cover half the distance between successive repeating stations. This is apparent from the fact that each repeating station is capable of non-directionally receiving the signals transmitted on frequency ,f2 from the mobile stations, and the latter can never be a distance from a repeating station greater than half the distance between each pair of adjacent repeating stations.

. It may happen that, because of local conditions, small portions of the area covered by the system may be in dead spots with respect to the nearest repeatingstations, particularly when using high frequency waves that are propagated only in substantially along a line of sight. Thus, referring to Fig. 1, let it be assumed that a portion il of the highway 20 runs through a narrow canyon in a direction at an angle to the line interconnecting the next adjacent repeating stations R2 and R3, and that because of this fact mobile units are unable to either transmit or receive to the nearest repeating station R3. Such a situation can be corrected by providing a special auxiliary repeating station 18 (Fig. 4) at a strategic point near the dead section ll. Thus, the station may be located at a high point in direct view of all parts of theroad section andfalso in direct view of the repeating station R3. This repeating station can comprise three interlocked repeaters 19,' 80 and 8|, generally similar to the units 36, 39 and 42 of the repeating stations RI, R2 and R3 of Figs. 1 and 3, but having special antenna equipment to meet the particular requirements.

Thus, the repeater 19 may have a bi-directional receiving antenna 82 adapted to receive signals on frequency f2 emanating from either the repeating station R2 or R3 and may repeat these signals on frequency f|`bidireotional1y from an antenna 89 into sectors 83 and 84, which include the dead section Il of the highway. Repeater 80 can have a bi-directional receiving antenna 85 adapted to receive signals on frequency f2 from a mobile station on the dead section Tl of the highway vand retransmit the signals bi-directionally from an antenna 90 on frequency fl into sectors 86 and8l, which include the repeating stations R2 and R3'respectively. The repeating unit 8| can have a receiving antenna 88 bi-directionally receiving signals on frequency f| from either the repeating station R2 or the repeating station R3, and repeat these signals over a bi-directional antenna 9| into the sectors 83 and 84 on frequency f2.

The equipment at an auxiliary repeating station can be modified to fit the particular requirements. Thus, in the particular instance shown (Fig. 1) in which the dead section 11 of the highway is relatively close to the repeating Sta- 10 tion R3, the antennas 82 (Fig. 5), 9ll'and 88 may be uni-directional instead of loi-directional, for communication only with the repeating station R3.

In some instances, vit is necessary only to boost the relatively low power transmitted from the mobile stations, and, in such a situation, the auxiliary repeating station might include only the single unit 8i), the signals emanating from the repeating station R3 being of sufficient power to be received directly on the mobile receiver.

As has been previously indicated, the receivers at allthe sub-stations are adapted to receive signals on both frequencies fl and f2. Several different types of receiving equipment can be used for this purpose. Two types having particular advantages in the present system are shown schematically in Figs. 7 and 8.

Fig. rl discloses a special super-heterodyne oircuit employing only a single oscillator, that can be used when the frequencies fl and f2 are properly chosen. Its operation depends upon having a relation between the two frequencies j| and f2 such that one local oscillator frequency beats with frequency f| to produce the desired intermediate frequency, and another, harmonically related, frequency of the local oscillator beats with the frequency f2 to produce the same intermediate frequency.

Thus, in Fig. '7, it is assumed that frequency fl is 149 mc., frequency f2 is 151 mc., and the intermediate frequency of the receiver is 49 mc. The receiver comprises as its essential elements the usual mixer 92, local oscillator 93, I. F.,

, amplifier 94, detector 95, and telephone 96. The

local oscillator 93 is of a common type producing both a fundamental and a harmonicY frequency, and in this instance, is designed to developed frequencies of mc. and 200 mc. The 100 mc. component beats with the 149 mc. Signal current to produce the intermediate frequency of 49 mc., and the 200 mc. component of the oscillator beats with the 151 mc. signal current to also produce the same intermediate frequency of 49 mc.

It is conceivable that a sub-station may sometimes simultaneously receive signals of both frequency fl and frequency f2, but differing slightly from the nominal frequencies by an amount such that they would produce, in the receiving circuit of Fig. 7, intermediate frequencies differing sufliciently to produce an audible whistle which might be objectionable. This effect can be eliminated by staggering the frequencies at different repeating stations suliciently to cause any beat note to be of frequency above the audible range or at least above the range transmitted by the audio systems of the receivers, and such a method of staggering the frequencies will be described later. However, it is also possible to' eliminate any audible whistles that might result from simultaneous reception of signals of frequencies fl and f2, by separately amplifying and detecting the signals in the receiver, and a circuit for accomplishing this is shown in Fig. 8.

The receiving system of Fig. 8 comprises a mixer 91, a local oscillator 98, an I. F. amplifier 99 tuned to 23 mc. and an I. F. amplifier |00 tuned to 25 mc., with separate detectors IGI and |02 connected thereto. The mixer 91 feeds into the two I. F. amplifiers and the separate detectors IDI and |02 feed into a common telephone |03. lAssuming that the frequency fl is 149 mc. and frequency f2 is `151 mc., the local oscillater 98 can have a frequency of 126 mc.,` which assay-is beats with the 149 mc. to produce an I. F. frequency of 23 mc. which is amplified by the I. li. amplifier 99, and beats with the frequency f2 of 151 mc. to produce an intermediate frequency of 25 mc., whic-h is separately amplified in the I. F. amplifier |00. The intermediate frequencies of 23 mc. and 25 me. are then separately detected in the separate detectors lOl and |02, and all frequencies other than the audio component are eliminated before the audio components are applied to the common telephone |03.

As has been previously indicated, whistles resulting from simultaneous reception by any receiver in the system of signals from two transmitters can produce an audible whistle o r beat note if both signals are received in sufficient strength and the carrier frequencies differ by an amount lying within the audio range. Thus, referring to Fig. 2, suppose that a signal originating at the mobile station MI on frequency f2 -is recei'ved-on `antenna 43 at repeating station Rl and is lretransmitted on frequency fl from antenna 44. The signal of frequency fl from antenna 44 is received on antenna 40 at repeating station R2 and retransmitted on frequency f2 from the unidirectional antenna 4l at repeating station R2. Because of the fact that antenna 4l is unidirectional, the eld strength of the signal of frequency ,f2 transmitted from repeating ystation R2 will be relatively weak at repeating station y Rl as compared to 'the field intensity of the signal of frequency f2 being received from the mobile sub-'station MI, so that it will not be strong enough to set up a singing circuit. However, even though the signal of vfrequency f2 from antenna 4| yat repeating station R2 is sufficiently weak at repeating station Rl to cause no trouble in other ways, it may be sufficiently strong to produce an audio beat note or whistle with the strong signal being received from the mobile station Ml. Possibility of such whistles can be eliminated in accordance with the present invention by staggering the frequencies kof the oscillators at the repeating stations with respect to the nominal frequencies fl and f2, so that the frequencies transmitted by valternate repeating stations 'will always differ from each other and from the signals transmitted vby the sub-stations, by an 'amount above the audio range of the receivers in the system. The result can be accom- .7.'

plished'by adjusting the transmitters at repeating stations RI and R2 to transmit at a frequency 10,000 cycles above the nominal value of frequencies fl and f2, and adjusting the transmitters at repeating station R3 to transmit at frequencies 10,000 cycles below the nominal value of frequenciesjl and f2. Regardless of the number of repeating stations in the system, the Same plan can be followed, namely, that of making the first two .repeating stations transmit at 10,000 cycles above the nominal values of frequencies fl and f2, making the next two repeating stations transmit at 10,000 cycles below the nominal values of frequencies ,fl and f2 and so on. There is illustrated schematically inFig. 6 a system including five repeating stations RI to R5, in which the first two stations RI and R2 transmit at frequencies of either .fl .plus 10,000 cycles, or f2 plus 10,000 cycles. The vnext two stations R3 and R4 transmit at frequencies of either fl minus 10,000 cycles or 1J2 minus 10,000 cycles, and the last station R5 transmits at either fl plus 10,000 cycles or f2 plus 10,000 cycles. It will be observed that with 'this system any pair of alternate repeating stations are always transmitting at lfrequencies means for 1'2 differing by 20,000 cycles from each other and by 10,000 cycles from the frequencies used by the sub-stations'. Hence, any beat notes :resulting are of either approximately 20,000 cycles or 10,000 cycles, which .are readily filtered out by the audio circuits ofthe receivers.

AIt has been previously explained that the antennas 43 and 44 at each repeating station and the antennas 45 and 48 at the stationary substations are preferably bi-directional for the sole purpose of increasing the efficiency of transmission and reception and that 'insofar asthe operation of the system is concerned, these antennas could be completely non-directional. Likewise, the antennas 29 and 3-I at the terminal stations X and Y have been shown as uni-directional for the sole purpose of increasing their efficiency in the useful directions. But these antennas likewise could be non-directional without interfering with the operation of the system. -As used in the claims, the 'words non-directional may include bi-directional transmission where the system covered by the claim, it is not material whether or not the transmission referred to as non-directional is completely non-directional. On the other hand, as used in the claims, the word vdirectional signifies an antenna structure that definitely .has a much lower efficiency of radiation or reception in `at least one direction than in another direction.

Although for the purpose of explaining the invention, a particular embodiment thereof has been shown and described, obvious modifications will occur to a person rskilled in the art, and I do Ynot desire to be limited to the exact detailsshown and described.

I claim:

1. In a radio communication system: a plurality of sub-stations distributed along a route of propagation, each sub-station including means for receiving signal-modulated wave energy in both a first and a second frequency band, and means for radiating signal-modulated wave en- -ergy 1in one of said vfrequency bands; and a plurality of repeating .stations distributed along-said route of propagation, each repeating station .in- 'cludingz a'iirst :means for directionally receiving signal-modulated wave energy within said frst band from one direction along lsaid route and directionally.retransmitting said signal modulation on wave energy in said second band in another direction along said route, a second means for direetionally receiving signal-modulated wave energy within said first band from said other direction along said route and directionally retransmitting signal-modulation on wave energy in said secondhand in said one direction, a third non-directionally receiving signal- `modulated wave energy in said second band and non-directionally retransmitting said signal.

modulation on wave energy in said first band,

and means responsive to the presence of output energy in any one of said receiving means for disabling the other two receiving means.

2. A. system as described Ain claim l, in which said sub-stations radiate signal-modulated energy in said second `frequency band.

3. A system as described in claim l, including an auxiliary repeating Ystation for communicatively coupling sub-stations along a portion of said route Ito one of Ysaid repeating stations, said auxiliary repeating station comprising: means for vreceiving signal-modulated wave energy within either band and retransmitting waive energy modulated with said signal in the other band.

13 4. A system as described in claim 1, in which said sub-stations transmit wave energy at a predetermined frequency within said second band and receive signal-modulated energy over the entire range of frequencies within said rst and second bands, and in which said repeating stations receive signal-modulated energy of all frequencies in said first and second bands and alternate repeating stations transmit at frequencies Within said bands displaced from each other by an amount in excess of the Width of the audio band passed by the receiving means in the system.

5. A system as described in claim 1, in which said sub-stations transmit at the mean frequency Within said second band and receive over the entire range of frequencies Within said :first and second bands, and in which said repeating stations receive over the entire range of frequencies in said first and second bands, and one of I' each pair of alternate repeating stations transmits at frequencies displaced in one direction from the mean frequency of the band by an amount in excess of the width of the audio band passed by the receiving means in the system, and the other of each pair of alternate repeating stations transmits at frequencies displaced in the opposite direction from the mean frequency of the band by an amount in excess of the width of the audio band passed by the receiving means in the system.

6. A system in accordance with claim 1 in Which: said signal modulations lie below a predetermined frequency; the frequency of said Wave energy transmitted by said first means differs from the frequency of the Wave energy received by said third means by a Value lying between said 14 predetermined frequency and the difference between the frequencies of the energies received by said first and third means respectively; and the frequency of said energy retransmitted by said third means differs by said value from the frequency of the energy received by said first means.

7. A system in accordance with claim 1 in which: the frequencies of said rst and second bands have substantially the same nominal value at all said repeating stations but the Waves radiated from alternate repeating stations are slightly displaced in opposite senses from said nominal value.

8. A system according to claim 7 in which the magnitude of said displacement exceeds the Width of the signal band with which said Wave energy is modulated.

HENRY T. WINCHEL.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,624,966 Morris Apr. 19, 1927 1,918,262 Goldsmith July 18, 1933 2,140,730 Batchelor Dec. 20, 1938 2,155,821 Goldsmith Apr. 25, 1939 2,183,741 Grundmann Dec. 19, 1939 2,186,980 Lowell Jan. 16, 1940 2,292,222 Haigis Aug. 4, 1942 2,487,513 Beverage Nov. 8, 1949 OTHER REFERENCES Electronics, May 1942, pp. 34 to 5l. 

